1. Field of the Invention
The present invention relates to the structure of a cemented carbide drill which is mainly adapted to drill steel members, and more particularly, it relates to the structure of a high-quality cemented carbide drill which is excellent in abrasion resistance and toughness.
2. Description of the Background Art
A drill is one of cutting tools for cutting steel members or the like. FIG. 1 shows an exemplary structure of a twist drill. This twist drill is formed by a cutting edge portion 1 for drilling and a shank portion 2, which is not concerned with cutting but mounted on a chuck, etc. of a cutting machine, such as a drilling machine, for discharging chips.
In general, such a drill is made of high-speed steel or cemented carbide. The high-speed steel, which is excellent in toughness, but inferior in abrasion resistance, is not suitable for high-speed cutting. On the other hand, the cemented carbide is excellent in abrasion resistance, accuracy required for a tool etc., but inferior in rupture strength to the high-speed steel. Therefore, if the cemented carbide is employed for a machine tool having low rigidity, for example, the tool may be broken.
Further, it is indispensable to prepare a drill core, which has a slow cutting speed and a large negative rake angle, from a strong cemented carbide material, i.e., a material having inferior abrasion resistance, in order to prevent chipping of the cutting edge. Thus, it has been extremely difficult to prevent agglutinative abrasion of a rake face provided in the inner peripheral portion of the drill and abrasion of the outer peripheral portion which has a higher cutting speed.
In order to improve the drill in these points, there have been proposed a method of coating the cutting edge portion of high-speed steel with a hard TiN layer, a method of preparing the cutting edge portion from cemented carbide and brazing the same, and the like.
In recent years, there have also been proposed a technique of brazing different types of cemented carbide materials (P30 and D30) for improving abrasion resistance and toughness as disclosed in Japanese Utility Model Laying-Open No. 58-143115(1983) or metallurgically integrating/joining the materials as disclosed in Japanese Utility Model Publication No. 62-46489(1987), a double-structure drill having a core and an outer peripheral portion which are formed of different types of cemented carbide materials in consideration of different characteristics required therefor as disclosed in Japanese Patent Laying-Open No. 62-218010(1987) or a method of forming such a double-structure drill by injection molding as disclosed in Japanese Patent Laying-Open No. 63-38501(1988) or 63-38502(1988), and the like. Further, Japanese Patent Laying-Open No. 62-292307(1987) discloses a technique of forming a drill from cement, in order to improve agglutination resistance of the drill.
The cutting edge portion and the shank portion of the drill are used under different load conditions. Therefore, the respective portions of the drill must have different characteristics. For example, abrasion resistance, agglutination resistance and the like are required for the tip of the cutting edge portion, while toughness is required for the shank portion in order to maintain strength of the tool. As to the tip of the cutting edge portion, further, different characteristics are required for the core and the outer peripheral portion, which are extremely different in cutting speed from each other.
In order to satisfy such complicated requirements for the characteristics of the drill, the cutting edge portion may be coated with a hard film. Once the hard film is separated, however, the coated drill is abruptly abraded. In general, the tip of an abraded spent drill is polished for re-usage. However, when the drill is subjected to re-grinding, the coating layer is inevitably separated at least from the front flank side, to damage most of the coating effect. In this case, therefore, it is necessary to re-coat the polished portion in order to effectuate original abrasion resistance. Consequently, the cost for the tool and the burden on tool management are increased under the present circumstances. As to the technique of brazing cemented carbide to the cutting edge portion, there has been such a disadvantage that the brazing method itself is essentially inferior in thermal strength and mechanical strength and not applicable to working of an unworkable material and deep hole drilling. Further, if cemented carbide is formed from coarse particles or highly bonded phases in order to improve toughness of the shank portion of the drill as recently proposed, strength of the material is reduced or distortion of the elastic limit is lowered to break the shank portion due to vibration of workpieces or instable rotation of the cutting machine during drilling operation.